The present invention relates to a solid electrolyte for an all-solid battery. In particular, it relates to a solid electrolyte for an all-solid thin-film lithium rechargeable battery.
According to development of portable devices such as personal computers and cellular phones in recent years, demands for a battery as a power source is increasingly growing. Especially, due to small atomic weight and high ionization potential of lithium, a lithium rechargeable battery has been actively studied in various fields as a promising battery capable of showing high energy density.
Since batteries for such applications adopt a liquid electrolyte, it is difficult to completely resolve problems such as leakage of the electrolyte. Further, due to its high energy density, the lithium rechargeable battery may possibly cause generation of heat if trouble is caused in the battery. For these reasons, the electrolyte is required to be nonflammable.
A solution to these problems is an all-solid battery using a solid electrolyte in place of the liquid electrolyte. The battery is made of solid components only, permitting improvement in battery reliability, as well as reduction in size and thickness of the battery. Thus, development of an all-solid lithium rechargeable battery using a solid electrolyte made of a nonflammable solid material has been longed for.
Known examples of the solid electrolyte for the all-solid lithium rechargeable battery include, for example, lithium halide, lithium nitride, oxyacid salt of lithium and their derivatives. In particular, U.S. Pat. No. 5,597,660 has reported that lithium phosphorus oxynitride (LixPOyNz: x=2.8, 3z+2y=7.8) obtained by introducing nitrogen (N) into lithium orthophosphate (Li3PO4) shows as considerably high Li ion conductivity as 1 to 2×10−6 S/cm in spite that it is an oxide-based material. Therefore, it has been adopted in an all-solid lithium rechargeable battery, especially in an all-solid thin-film lithium rechargeable battery, used as a power source for microelectronic devices such as IC cards and electronic tags.
However, if the lithium phosphorus oxynitride (LixPOyNx) adopted as the solid electrolyte in the above-described patent publication is left in humid atmosphere, it reacts with moisture to deteriorate, considerably decreasing its ion conductivity. A cause of the deterioration is that a phosphorus atom (P) is reduced from +5 to decompose the lithium phosphorus oxynitride.
If such deterioration occurs, an all-solid lithium rechargeable battery decreases in electrochemical characteristics such as charge-discharge, storage and charge-discharge cycle characteristics. Accordingly, an object of the present invention is to provide a solid electrolyte suppressing the reduction of phosphorus atoms in lithium phosphorus oxynitride as much as possible to resist the deterioration.